Automatic fidelity control circuit



Jan.'1l, 1938 R. A. BRADEN 2,104,788

AUTOMATIC FIDELITY CONTROL CIRCUIT Original Filed July 29, 1955 \IIL 7) I v I some: a; s r 0i i :E

SIGNALS I T+ RECT/F/ER 70 A. F.

WEAK SIGNALS I lNTERMED/ATE VALUES STRONG S/GNALS FREOUEN CY INVENTOR RENE A. BRADEN BY I ATTORN EY Patented Jan. 11, 1938 PATENT OFFICE AUTOMATIC FIDELITY CONTROL GIRCUI Rene A. Braden, Collingswood, N. J assignor to Radio Corporation of America,- a corporation of Delaware Original application July 29, 1933, Serial No. 682,743. Divided and this application June 17, 1936, Serial No. 85,644 I 8 Claims.

My present invention relates to fidelity control arrangements for radio frequency signalling systems, and more particularly to automatic fidelity control circuits for radio receivers; the present application being a division of my application Serial No. 682,743, filed July 29, 1933.

It is one of the primary objects of my present invention to provide a method of automatically controlling the selectivity of a radio frequency signalling system whereby the system ismore selective on weak signals than on strong ones with the result that on strong local stations the selectivity is least, but on weak stations it is most.

Another important object of the present invention is to provide an automatic fidelity control for a radio receiver of the type wherein the resonance curve of a tunable network is automatically varied in response to the variation in amplitude of the received signal energy.

Another important object of the present invention is to provide an automatic selectivity control circuit for a radio receiver wherein the rectified signal energy of the receiver is utilized to vary the high frequency response characteristic of a resonant circuit preceding the receiver rectifier.

Still another object of the present invention is to provide a fidelity control arrangement for a radio receiver, the latter operating on a band pass network preceding a rectifier in such a manner that the resonance curve of the network is automatically varied to have a single resonance peak when weak, or distant, station signals are received, and a double peak when strong, or local, stations are received.

Still other objects of the present invention are to improve generally the efiiciency of radio receiving systems, and particularly to provide automatic fidelity control circuits for radio receivers which are not only eflicient and reliable in operation, but economically constructed and assembled in radio receivers.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of, operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into efiect.

In the drawing: 7

Fig. 1 shows a form of the invention, and

Fig. 2 is a graphic representation of the operation of the circuit in Fig. 1.

I There is shown in Fig. 1 an electrical method of selectivity control in a radio receiver. In this figure the source of signal energy is coupled to the input electrodes of the screen grid tube 6, and the input circuit of the rectifier 8 is coupled to the anode circuit of the screen grid tube 1. The anode circuit of screen grid tube 6 is coupled to theinput electrodes of tube 1 through a tuned circuit I, the anode of tube 6 being connected to the control grid of tube 1 through a condenser C. A second tuned circuit II is connected to the screen grid electrode of tube 6', as well as to the anode of a control tube It. The screen grid of tube 6 and the anode of tube It have positive potentials applied thereto from a source B (not shown) through the coil of the resonant circuit II.

I The cathode of control tube It is connected to the common lead connecting the cathodes of tubes 6' and I, while the control grid of tube I6 is adjustably connected, as by a lead 9, to the negative side of resistor R. It is to be clearly understood that the rectifier 8 may be the usual detector of the receiver, or an auxiliary rectifier deriving its signal energy input from the input to the said usual detector.

The resonant circuit II and the circuit I are both tuned to the desired carrier frequency, the circuit II reacting so as to cause the over-all resonance curve of the network between amplifier tubes 6' and "I to be like the curve A in Fig. 2. If the bias on the grid of tube I6 is reduced so as to reduce the R of the tube, the effect of circuit II is reduced, and the resonance curve becomes sharper, and the gain is increased, as shown by the curve D in Fig. 2. By controlling the negative bias on the grid of tube IS in acordance with the intensity of the rectifier plate current, the selectivity of the receivingsystem shown in Fig. 1 can be made to increase with decreasing signal strength. The curves between curves A and D of Fig. 2 show intermediate signal values.

Where, for example, the network 6'-I-l is a stage of intermediate, frequency amplification, the circuit I is given a sharp resonance curve. The circuit II, which is tuned to the carrier, pushes down the peak of this resonance curve, making a broad or double-peaked curve, and reducing the gain. With low bias on the control tube Hi the tuned circuit 11 is almost short-circuited, and the efiect of this circuit is very small. With high bias on tube l6, the circuit II has its maximum effect, and hence provides a network of optimum fidelity when it is receiving strong signals.

The action in this case of Fig. 1 is similar to that described in my application Serial No. 517,154, filed February 20, 1931 patented February 13, 1934 as U. S. Patent 1,947,184 in connection with Figs. 3 and 4 of that patent. In that patent the output circuit had no frequency characteristics, and so the over-all curve had an intermediate valley. In the present case, the

output circuit has a resonance curve, and the tuned circuit in the screen operates to reduce the gain a large amount at resonance, and smaller amounts off resonance. In this Way the gain curve D (Fig. 2) of the amplifier is operated on to produce the curves below it corresponding to various shunt resistances across the tuned circuit II. Curve A provides compensation for loss of high modulation frequencies in other radio frequency and intermediate frequency stages not provided with fidelity control.

The. selectivity control arrangement of Fig. 1 operates in the manner shown by the curves. of Fig. 2. The selectivity control operation which is secured in this circuit can, in addition to the explanation given above, be analyzed as that of a reversed feed-back circuit, A signal impressed on the control grid of tube 6' causes alternating current to flow in the screen and plate circuits. Screen current flows through the -im-, pedance of the auxiliary tuned circuit II producing alternating voltage between screen and cathode which is similar to the signal voltage but reversed in phase. The screen then acts like a second control grid, and causes an alternating plate current to flow which adds to that produced initially by the control grid signal. At the resonant frequency of the tuned circuits these two alternating currents are in phase opposition, that produced by the screen voltage being the smaller, and the total output is less than it would be if there were no impedance in the screen circuit- At frequencies above and below resonance the screen voltage decreases, because the tuned circuit impedance is smaller, and, consequently, there is less reduction of the output current. At frequencies sufiiciently far from the resonant frequency the screen tuned circuit has no effect. In this way the resonance curve A shown in Fig. 2 is produced.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In combination with a source of signals, a transmission tube having in addition to input and output electrodes an auxiliary cold electrode disposedin the electron stream to the anode, a circuit tuned to a desired signal frequency .connected to the said output electrodes, said tuned circuit having a resonance curve characteristic of a predetermined shape, a reactive network tuned to said desired frequency connecting said auxiliary electrode and the tube cathode, an impedance in shunt with the reactive network, and means, responsive to variations in signal amplitude, for changing the magnitude of said shunt impedance thereby to control the eifect of said reactive network on said characteristic shape. a

2. In combination with a source of signals, 21.

transmission tube having in addition to input and output electrodes an auxiliary cold electrode disposed in the electron stream to the anode, a circuit tuned to a desired signal frequency connected to the said output electrodes, said tuned circuit having a resonance curve characteristic of a predetermined shape, a reactive network tuned to said desired frequency connecting said auxiliary electrode and the tube cathode, an impedance provided by the cathode to plate impedance of an electron discharge tube in shunt with the reactive network, and means, responsive to variations in'signal amplitude, for changing the magnitude of said shunt impedance thereby to control the effect of said reactive network on said characteristic shape.

3. Ina screen grid amplifier having a signal input circuit and an output circuit tuned to a desired carrier frequency, the output circuit having a resonance curve characteristic of a predetermined shape, an alternating current path between the screen grid and cathode of the amplifier which includes a circuit tuned to said carrier frequency whereby the shape of said characteristic is changed, and means, responsive to the carrier amplitude, for controlling the eifect of the second tuned circuit on said shape.

4. In a wave transmission system, a tube of the screen grid type, an output circuit for the tube having a relatively sharp resonance curve characteristic, reactive means connected between the screen and cathode of the tube and adapted to broaden said characteristic, and means responsive to a decrease in wave amplitude for rendering said reactive means ineffective.

5. In a wave transmission system, a tube of the screen grid type, anoutput circuit for the tube having a relatively sharp resonance curve characteristic, reactive means comprising a circuit tuned to the wave frequency of said output circuit connected between the screen and cathode of the tube and adapted to broaden said characteristic, and means responsive to a decrease in wave amplitude for rendering said reactive means inefifective.

6. In a wave transmission system, a tube of the screen grid type, an output circuit for the tube having a relatively sharp resonance curve characteristic, reactive means connected between the screen and cathode of the tube and adapted to broaden said characteristic, means responsive to a decrease in wave amplitude for rendering said reactive means ineffective, said last means including a tube having its internal impedance connected across the reactive means, a wave rectifier arranged to produce a direct current voltage varying directly in magnitude with said amplitude, and means to control the said impedance value with said direct current voltage.

7. In a wave transmission system, a tube having a wave input circuit and a Wave output circuit, the latter being tuned to a desired wave frequency, means, responsive to waves impressed on said input circuit, producing currents of said wave frequency in said output circuit but of reversed phase with respect to wave currents in said input circuit, and means, responsive to a decrease in wave amplitude at the input circuit, for controlling the first means in a sense to reduce the;

reversed phase current in said output circuit.

8. In a wave transmission system, a tube having a wave input circuit and. a wave output circuit, the latter being tuned to a desired wave frequency, means, responsive to waves impressed on said input circuit, producing currents of said i5 first means including an alternating current circuit, an electrode within said tube disposed in the space current to said output circuit, and said electrode being connected to said alternating curwave frequency in said output circuit but of reversed phase with respect to wave currents in said input circuit, means, responsive to a decrease in wave amplitude at the input circuit, for controlling the first means in a sense to reduce the reversed phase current in said output circuit, said rent circuit. 

